1. Field of the Invention
The present invention relates to radiographic imaging apparatus, method and program for obtaining a plurality of images at a plurality of radiation source positions, to achieve, for example, tomosynthesis imaging to generate tomographic images.
2. Description of the Related Art
In recent years, tomosynthesis imaging has been proposed with respect to X-ray imaging apparatuses to more closely observe an affected part of the body. In the tomosynthesis imaging, imaging operations are carried out by applying the X-ray to a subject from different angles with moving the X-ray tube, and the thus obtained images are added up to provide an image in which a desired slice is emphasized. In the tomosynthesis imaging, the X-ray tube may be moved in parallel with the X-ray detector or may be moved in a circular or ellipsoidal arc, depending on characteristics of the imaging apparatus and necessary tomographic images, to obtain images of the subject imaged with different irradiation angles, and these images are reconstructed to generate tomographic images.
In the case where such tomosynthesis imaging is performed, it is necessary to align the images obtained through the imaging operations to reconstruct the images. For this purpose, a technique which involves calculating the positions of the X-ray tube during the imaging operations (which will hereinafter be referred to as “radiation source positions”) by equally dividing the range of movement of the X-ray tube by the number of imaging operations (number of shots), and reconstructing the images with using information of the calculated radiation source positions has been proposed.
This technique, however, has difficulty in accurately moving the X-ray tube to the calculated radiation source positions due to influences of vibration during imaging, mechanical misalignment, etc., and therefore the radiation source positions during imaging are displaced from the calculated radiation source positions. Due to this displacement, it is impossible to achieve accurate alignment of projection positions of the object, resulting in degradation of image quality of the tomographic images.
Therefore, it is practiced during tomosynthesis imaging to place markers on a subject or on an imaging table on which the subject is placed, and imaging the markers together with the subject to obtain images containing marker images (see Japanese Unexamined Patent Publication No. 2005-021345, U.S. Patent Application Publication No. 20040252811, and U.S. Pat. No. 6,196,715, which will hereinafter be referred to as Patent Documents 1 to 3). According to the techniques disclosed in Patent Documents 1 to 3, an accurate radiation source position for each image is calculated with using positional information of a phantom containing the markers, and the images are reconstructed with using the calculated radiation source positions to eliminate influence of the displacement of the radiation source positions. Further, a technique to simultaneously estimate a displacement parameter of a support position and an imaging position has been proposed with taking displacement of the position of the support containing a plurality of markers into account (see U.S. Patent Application Publication No. 20020131559, which will hereinafter be referred to as Patent Document 4).
On the other hand, a technique to correct for positional displacement between a plurality of images with using amounts of shift of positions of marker images between the images, without using positional information of the markers, has been proposed (see U.S. Pat. No. 6,960,020, which will hereinafter be referred to as Patent Document 5). Now, the technique disclosed in Patent Document 5 is described.
FIG. 9 is a diagram for explaining how alignment is achieved in the technique disclosed in Patent Document 5. In FIG. 9, a direction parallel to the movement path of the X-ray tube is referred to as the x-direction, a direction perpendicular to the movement path of the X-ray tube is referred to as the z-direction, and a direction perpendicular to the plane of the drawing is referred to as the y-direction. As shown in FIG. 9, when the X-ray tube is moved from a radiation source position S11 to a radiation source position S12 by an amount Δxs, a marker image of a marker M0 placed on a imaging table top 104 is shifted by an amount Δxm, and a projection position of an object T0, which is the object of reconstruction, in a subject 102 is shifted by an amount Δxt. Assuming here that a distance between the X-ray tube and the detection plane of a detector 114 (i.e., a radiation source distance) is sz, a distance between the detection plane of the detector 114 and a plane in which the object T0 is present is tz, and a distance between the detection plane of the detector 114 and the top surface of the imaging table top 104 is mz, then, the amount of movement Δxs of the radiation source position is expressed by Equation (1) below with using the amount of shift Δxm of the marker image, the radiation source distance sz and the distance mz:Δxs=Δxm×(sz−mz)/mz  (1)
Further, the amount of shift Δxt of the projection position of the object is expressed by Equation (2) below with using the amount of movement Δxs of the radiation source position, the radiation source distance sz and the distance tz:Δxt=Δxs×tz/(sz−tz)  (2)
According to Equations (1) and (2), the amount of shift Δxt of the projection position of the object is expressed by Equation (3) below with using the amount of shift Δxm of the marker image, the radiation source distance sz, the distance mz and the distance tz:Δxt=Δxm×(tz/mz)×(sz−mz)/(sz−tz)  (3)
By calculating the amount of shift Δxt of each projection position of the object in this manner, alignment of the projection positions of the object in the images can be achieved to reconstruct the tomographic images.
However, the techniques disclosed in Patent Document 1 to 3 use the positions of the markers as known information to calculate the radiation source positions. Therefore, in order to find accurate radiation source positions, it is necessary that the phantom containing the markers is accurately made, with the markers being accurately placed at predetermined positions in the phantom. Although accurate positional information of the markers can be obtained by fixing the markers to a fixture, such a fixture is large to some extent, and thus is not suitable for imaging various subjects. Further, use of a marker fixture limits positions of the markers, and therefore, in the case where a radiation field aperture is used during imaging, it may be impossible to position the markers within the radiation field. On the other hand, the technique disclosed in Patent Document 4 uses relative positions of the markers in the support as known information, and it is necessary to accurately position the markers in the support.
Further, the technique disclosed in Patent Document 5 assumes that the movement path of the X-ray tube is always parallel to the detection plane of the detector. Therefore, accurate alignment cannot be achieved when the radiation source positions for obtaining the images are not on a movement path that is parallel to the detection plane of the detector.